1. Field of the Invention
The present general inventive concept relates to an inkjet printhead, and more particularly, to an inkjet printhead that uses non-aqueous ink.
2. Description of the Related Art
An inkjet printhead is a device that prints a predetermined color image by ejecting minute droplets of ink on desired areas of a printing medium. Inkjet printheads can be generally classified into two types according to the ejection mechanism of ink droplets. The first type is a thermal inkjet printhead that ejects ink droplets using the expansion force of ink bubbles created using a heat source, and the second type is a piezoelectric inkjet printhead that ejects inkjet droplets using a pressure created by the deformation of a piezoelectric element.
FIG. 1 is a schematic cross-sectional view of a piezoelectric inkjet printhead as an example of a conventional inkjet printhead. Referring to FIG. 1, a flow channel plate 10 includes a manifold 11, a plurality of restrictors 12, and a plurality of pressure chambers 13, which constitute an ink channel. A vibration plate 20 that is deformed due to driving of piezoelectric actuators 40 is combined with an upper surface of the flow channel plate 10. A nozzle plate 30 having a plurality of nozzles 31 is combined with a lower surface of the flow channel plate 10. The flow channel plate 10 and the vibration plate 20 can be formed as one unit, and also the flow channel plate 10 and the nozzle plate 30 can be formed as one unit. The flow channel plate 10, the nozzle plate 30, and the vibration plate 20 are usually formed of silicon.
The manifold 11 is a path for supplying ink from an ink tank (not illustrated) to the pressure chambers 13, and the restrictors 12 are paths for supplying ink to the pressure chambers 13 from the manifold 11. The pressure chambers 13 are filled with ink to be ejected, and are arranged on one side or both sides of the manifold 11. The nozzles 31 are formed through the nozzle plate 30 and are connected to the pressure chambers 13. The vibration plate 20 is formed on an upper surface of the flow channel plate 10 to cover the pressure chambers 13. The vibration plate 20 is deformed due to the driving of the piezoelectric actuators 40 and provides pressure for the pressure chambers 13 to eject ink. Each of the piezoelectric actuators 40 includes a lower electrode 41, a piezoelectric film 42, and an upper electrode 43 sequentially formed on the vibration plate 20.
In an inkjet printhead having the above structure, an ink-philic coating film 34 formed of thermally oxidized silicon is formed on inner walls of the nozzles 31 and inner walls of the ink channel. The ink channel is a path for ink flow and includes the pressure chambers 13, the restrictors 12, and the manifold 11. If the inner walls of the nozzles 31 and the ink channel have an ink-philic property, a contact angle with respect to ink is reduced, and thus, capillary force increases. Therefore, the time required for refilling ink into the pressure chambers 13 is reduced and ejection frequency can be increased. An ink-phobic coating film 38 is formed on an external surface of the nozzle plate 30. The ink-phobic coating film 38 can be formed of perfluorinated silane, which is a well known material that can minimize ink-wetting by reducing surface energy of the nozzle plate 30. If the external surface of the nozzle plate 30 has an ink-phobic property, that is, a non-wetting property, the ink-wetting at the surface of the nozzle plate 30 can be prevented, and thus, straightness of ink droplets can be ensured.
Conventional inkjet printheads mainly use aqueous ink. When aqueous ink is used in an inkjet printhead having the above structure, since the inner walls of the nozzles 31 and the ink channel have an ink-philic property, that is, a high ink wetting property, ink refill can be smoothly achieved, and the performance of the inkjet printhead can be improved since air trapping on the inner walls of the ink channel is prevented.
Recently, the application of inkjet technology to various industrial fields such as display apparatuses, radio frequency identifications (RFID), or bio-chips is being actively studied. As a result, the development of non-aqueous ink besides the conventional aqueous ink is being accelerated.
However, if non-aqueous ink is used in a conventional inkjet printhead, the inner walls of the nozzles 31 and the ink channel become contaminated by ink residues, such as a dispersing agent or a pigment. That is, since the non-aqueous ink has a low surface tension compared to the conventional aqueous ink, the non-aqueous ink can relatively easily wet the inner walls of the nozzles 31 and the ink channel. Also, the non-aqueous ink has a high vapor pressure, since the non-aqueous ink easily vaporizes. Accordingly, the dispersing agent or the pigment in the non-aqueous ink strongly combines with oxidized silicon, which has a high surface energy, and as a result, the residue such as the dispersing agent or the pigment is adsorbed on the inner walls of the nozzles 31 and the ink channel. Also, since the non-aqueous ink has a high vapor pressure compared to the aqueous ink, ink evaporation actively occurs at a meniscus portion of ink that contacts the air, and as a result, the ink residue is adsorbed on the walls of the nozzles 31.
FIGS. 2 and 3 respectively are photo images of an inner wall of a nozzle and an inner wall of a restrictor, which are contaminated by residues when non-aqueous ink is used in an inkjet printhead of FIG. 1. However, a method of cleaning the inner walls of the nozzles and the ink channel when the inner walls of the nozzles and the ink channel are contaminated has not yet been developed.